Partial coronary occlusion was evaluated in open chest animals by recording epicardial and endocardial electrograms from the area of distribution of the occluded artery. Radioactive microsphere measurements of regional coronary flow were made before, during, and two to four minutes after partial coronary occlusion. Finally, animals subjected to three hours of partial coronary occlusion were killed two weeks later for pathological examination. As partial coronary occlusion is gradually increased, endocardial S-T segment elevation occurs at a higher distal coronary pressure than does epicardial S-T segment elevation. Coronary occlusion was adjusted to effect endocardial without epicardial S-T elevation at a distal coronary pressure of 40-50 mm. Hg.